Onset Potential for Electrolyte Oxidation and Ni-Rich Cathode Degradation in Lithium-Ion Batteries

High-capacity Ni-rich layered metal oxide cathodes are highly desirable to increase the energy density of lithium-ion batteries. However, these materials suffer from poor cycling performance, which is exacerbated by increased cell voltage. We demonstrate here the detrimental effect of ethylene carbonate (EC), a core component in conventional electrolytes, when NMC811 (LiNi0.8Mn0.1Co0.1O2) is charged above 4.4 V vs Li/Li+—the onset potential for lattice oxygen release. Oxygen loss is enhanced by EC-containing electrolytes—compared to EC-free—and correlates with more electrolyte oxidation/breakdown and cathode surface degradation, which increase concurrently above 4.4 V. In contrast, NMC111 (LiNi0.33Mn0.33Co0.33O2), which does not release oxygen up to 4.6 V, shows a similar extent of degradation irrespective of the electrolyte. This work highlights the incompatibility between conventional EC-based electrolytes and Ni-rich cathodes (more generally, cathodes that release lattice oxygen such as Li-/Mn-rich and disordered rocksalt cathodes) and motivates further work on wider classes of electrolytes and additives.

dried at 120 °C for at least 12 h under dynamic vacuum before being transferred to an Ar filled glove box (<0.5 ppm O2 and H2O, MBraun).

Full cell experiments
Coin cells (2032-type; Hohsen) were assembled with a NMC cathode versus a LTO anode with Three-electrode PAT cells (EL-Cell) were assembled with 18 mm diameter cathode and anode, a glass fiber separator (260 μm thickness, grade GF/A) soaked in 100 μL of various electrolytes, and a lithium metal ring electrode set in an insulation sleeve (EL-Cell).
After assembly the cells were cycled in a 25 °C environmental chamber with a Biologic VMP3 potentiostat or BCS 805 series battery cycler. The cycling protocol involved a C/20 (assuming a practical capacity of 147 mAh g -1 NMC for NMC111 and 185 mAh g -1 NMC for NMC811) charge to various upper cutoff voltages (UCVs) -2.75, 2.85, 2.95, or 3.05 V (vs. LTO)a 60 h voltage hold (VH) at the UCV, and a C/20 discharge to 1.45 V. The UCVs correspond to NMC potentials of 4.3, 4.4, 4.5, and 4.6 V vs Li/Li + , respectively, since the potential of the LTO intercalation plateau is at 1.55 V vs Li/Li + (refer to Figure S2). Two or more cells were evaluated for each condition to ensure reproducibility.

Impedance measurements
After the VH protocol the potential-dependent impedance of the NMC cathode was measured in three-electrode PAT cells. The NMC cathode was charged at C/20 to various potentials vs. the Li/Li + reference electrode -3.8, 4.1, 4.3, 4.5, 4.6 V for NMC111 and NMC811, and also 4.7, 4.8, and 4.9 V for NMC111held at each potential for 1 h to reach a steady-state, and left to rest at OCP for 1 h before the electrochemical impedance spectroscopy (EIS) measurement.
EIS was conducted with a Biologic VMP3 potentiostat in a frequency range of 500 kHz to 10 mHz with an AC voltage perturbation of 5 mV. The SOC of NMC at each potential was calculated based on the charge passed and a theoretical capacity of 277.9 mAh g -1 NMC for NMC111 and 275.5 mAh g -1 NMC for NMC811.
Online electrochemical mass spectrometry (OEMS) The OEMS system and the process used to calibrate the MS has been described in detail in ref. 3

Surface area analysis
The surface area of the NMC powders was determined by nitrogen gas physisorption at 77 K, measuring isothermally at 10 points between 0.07 ≤ p/p0 ≤ 0.30 (3Flex, Micromeritics).

Pristine electrolyte characterization
The water content of the pristine electrolytes was measured by Karl Fischer titration (899 Coulometer, Metrohm). Pristine electrolyte was prepared for NMR analysis by pipetting 40 μL of the electrolyte solution into 0.7 mL of DMSO-d6 (99.9 atom % D, 99 % CP; Sigma-Aldrich), which was transferred to an airtight NMR tube fitted with a Young's tap.

Sample preparation for post-mortem characterization
After the cycling protocol the NMC/LTO coin cells were disassembled in an Ar filled glove box. The separator was extracted and soaked in 0.7 mL of DMSO-d6 for 5

ICP-OES
Elemental analysis was performed using inductively coupled optical emission plasma spectroscopy (ICP-OES; Thermoscientific) calibrated with standards prepared from an ICP multi-element solution (VWR, Aristar).      The proposed reactions for the chemical oxidation of EC and EMC are shown in Scheme S1 and S2, respectively, below.
Scheme S1. Proposed reaction for the chemical oxidation of EC. 4 Scheme S2. Proposed reaction for the chemical oxidation of EMC. 3 These reactions reveal that the expected O2:CO2 mole ratio is 1:1 for both EC and EMC.
Therefore, the quantity of CO2 evolved with EC electrolyte and EMC electrolyte can be directly compared as an indicator for the quantity of (reactive) lattice oxygen release. 3 For CO2 we measure an EC/EMC fraction of 1.9 (calculated from the data in Figure 1e), suggesting that chemical oxidation of the electrolyte solvent is approx. 1.9 times higher with EC compared to EMC.
For CO, these reactions reveal that the O2:CO ratio is 2:1 for EC and 1:1 for EMC. In this case, the reaction stoichiometry needs to be taken into account before drawing conclusions about the relative amount of (reactive) oxygen release. Specifically, for the same amount of O2 release, we would expect the ratio of CO evolved for EC relative to EMC to be 0.5i.e. half as much CO would be evolved with EC electrolyte compared to EMC electrolyte. Instead, we measure an EC/EMC fraction of 0.8 (calculated from the data in Figure 1d). This indicates that the chemical oxidation of the electrolyte solvent is approx. 1.6 times higher (0.8/0.5 = 1.6) with EC compared to EMC.
Therefore, the quantity of CO and CO2 evolved (Figure 1d-e) both indicate that the chemical oxidation of the electrolyte solvent is higher with EC compared to EMC, by a factor of approx. 1.6-1.9 times.  Table S3.  The chemical shift labels in black are also present in the pristine electrolyte, while red correspond to signals that arise from electrolyte degradation.